PURPOSE: To enable highly accurate measurement even when an object to be inspected rotates a high speed, by arranging two UT sensors at equal distance points in the direction opposite by 180° to each other from the center as given when the eccentricity is zero.

CONSTITUTION: Two UT sensors 2 are arranged as opposed to each other at equal distance points in the direction opposite by 180° to each other from the point as given when the eccentricity is zero on the plane vertical to the axis of an object 1 to be inspected. The received signal from the UT sensor 2 is amplified with a preamplifier 4, sent to a converter/amplifier 5 to be converted and amplified and then, the signal is sent to a microcomputer 7 through an interface 6. The microcomputer 7 performs a computation processing and outputs the results to a display 8 such as CRT, a memory medium such as floppy disk 9 or a recorder such as a plotter (printer) 10.